<!doctype html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style type="text/css">
		#box{width: 200px;
			 height: 100px;
			 background:red;}
		#son{height: 20%;
			background: #0000FF;}
	</style>
</head>
<body>
	<div id="box">
		<div id="son"></div>
	</div>
</body>
</html>
